Power calculation

I am working on a study that aims to assess predictors of insufficient food intake among elderly patients. In the study I am evaluating (9) predictors. Regarding calculating the sample size, I decided to use the rule of thumb, but I am confused between to references that pointed out how to apply it:

-The first one suggested in their multivariate text that the minimum number of subjects for each predictor or independent variable in a regression analysis should be 5-to-1.